There is a way, but it requires planning or *cough* paying.
1) Changing Faith is a thing, which it actually trade a few hundreds of Critical stat to Attack stat. (assuming you already Max.Limit Break a hero, the changes won't affect your % much)
2) Changing Equipment, for Shadow, it is still advisable to get 'of Might' set due to the availability of Attack and Critical stats, so now, you may try to Reset the enhancement or to try to get new equipment with same suffix to enhance again.
3) Changing Runes, There are at least TWO combination for every stats. Either Focus rune of Attack or Fury rune of Critical. So you can gauge on this too.
Focus rune of Attack : 20% Critical with around 1400 Attack (assuming all 3* rarity with maxed upgrade)
Fury rune of Critical : 20% Attack with around 1000 Critical (same as above)
4) Familiarizing Aura, This one is tricky, but still do-able, it requires you to UNDERSTAND all the auras, or at least those that plays a part in your strategy, I can remember Winry Marskman, Izumi Magebane, Cybella Sharpshooter for now, these three said aura was to Buff Everyone in aura of Critical stat, so you can also count them in the final calculation.
Good luck, and if there was mistakes in above statements, please to point it out, I would like to know if myself was also right on my view. Thanks